GitLab

This CL implements a new tool called mb (//tools/mb) that wraps both GYP
and GN to implement the 'gen' and 'analyze' steps on the bots. It
serves three purposes:

1. To move the decision to use GYP or GN to generate the build files to
   a configuration stored in the repo (and to hide it from the bots).
2. To move which flags are used to a configuration stored in the repo,
   so that we can version the flags alongside the code, and change the
   flags without needing to change things in the build repo.
3. To consolidate a list of all of the different build configs we need
   to support and verify for the GYP->GN migration.

For more details on the above, see //tools/md/README.md and the linked
docs.

R=scottmg@chromium.org, brettw@chromium.org
BUG=466436

Review URL: https://codereview.chromium.org/1062613004

Cr-Commit-Position: refs/heads/master@{#324231}